Space Shuttle's Rare Landing Approach to be Visible Across United States

Skywatcher Dr. Dale Ireland took this photo of the space shuttle Atlantisduring its STS-79 mission as it re-entered the Earth's atmosphere on Sept. 26, 1996 from Silverdale, Washington.Credit: Dale Ireland/www.drdale.com

NASA's space shuttle Discovery will attempt a
return to Florida's Kennedy Space Center Monday morning by taking a path across
the contiguous United States, giving early-bird skywatchers a chance to see ?
and hear ? the spacecraft as it streaks across the sky on the way to
landing.

The space
shuttle might be visible to keen eyes along the flight path, which runs
from the northwest toward the southeast. And many people are likely to hear its
sonic boom.

It is only the second time since NASA's tragic
2003 loss of the shuttle Columbia than returning shuttle will fly over the
continental United States, and is also expected to be the last time, according
to entry flight director Bryan Lunney.

NASA's three remaining shuttle missions after
Discovery's are in the summer, when NASA tends to avoid this specific landing
profile ? called a "descending node" ? in order prevent a shuttle
from flying through high-altitude noctilucent clouds, which are common in the
summer months, he said. The shuttles usually approach Florida from the
southwest, flying over Central America on the way.

The shuttle Discovery ? which undocked from
the International Space Station on Saturday morning -- is aiming for a
touchdown at NASA's
prime landing site in Florida. There will be two possibilities for a
Florida landing, on orbits 222 and, if necessary, orbit 223.

If all goes according to plan, Discovery will
fire its twin braking rockets at 7:43:20 a.m. EDT over the Indian Ocean,
just to the south of the French Southern and Antarctic Lands (a group of
volcanic islands southeast of Africa), for 3 minutes and 11 seconds, slowing
the shuttle down for its de-orbiting and a gradual slide back to Earth.

At 8:16:59 a.m. EDT, the orbiter will arrive
at that point where it will begin to encounter the first effects of the Earth's
atmosphere at an altitude of 399,800 feet.

Where to look

On Orbit 222, Discovery will cross the
western coast of North America at 8:25 a.m. EDT (5:25 a.m. PDT), south of
the Queen Charlotte Islands of western Canada. [Graphic:
Discovery's landing paths across the U.S.]

From Vancouver, British Columbia prospective
shuttle watchers will see Discovery reach a maximum altitude of 18-degrees
above the northern horizon at 5:25:31 a.m. PDT. As a reference, your
clenched fist held at arm's length measures roughly 10-degrees in width, so as
seen from Vancouver, the Shuttle will appear to reach nearly "two
fists" above the northern horizon.

Traveling on a southeast trajectory, the
orbiter will then pass over northeast Montana.

From Glendrive, viewers will see
the shuttle reaching a maximum altitude of 50-degrees above the northeast
horizon at 6:28:51 a.m. MDT. Just over a minute later at 7:30:08 a.m. CDT,
Discovery will be passing almost directly over Pierre, South Dakota.

From there, the shuttle will reach a maximum
altitude of 82-degrees up in the northeast. After about another three more
minutes, it will be racing over the heart of Missouri, between Kansas City and
St. Louis. From Kansas City Discovery will be 25-degrees high above the
north-northeast horizon at 7:32:11 a.m. CDT, while from St. Louis it will
passing 21-degrees above the southwest horizon at 7:33:12 a.m. CDT.

About a minute later it will be over the
Arkansas-Tennessee border to the east of Memphis, where Discovery will reach an
altitude of 33-degrees above the north-northeast horizon at 7:34:13 a.m. CDT.

At 8:44:55 a.m. EDT, the shuttle will
decelerate to two and a half times the speed of sound (mach 2.5), dropping
to an altitude of 82,200-feet just to the northwest of Cape Canaveral.
Touchdown is scheduled for 8:51:22 a.m. EDT.

Should Discovery be waved off on this first
attempt, a second attempt will be made on orbit 223.

On this track, the shuttle would reach the
Washington coast south of the Seattle-Tacoma area shortly after 7:00 a.m.
PDT.

From Seattle, viewers should look 44-degrees
above the southwest horizon at 7:00:44 a.m. PDT. Discovery will then pass
over Southwest Wyoming, reaching an altitude of 48-degrees up in the
north-northeast at 8:03:48 a.m. MDT as seen from Rock Springs. It will then
pass to the north of Denver, Colorado at 8:04:49 a.m. MDT, at an altitude of
45-degrees and four minutes later it will be over Arkansas, making a flyover
almost directly above Little Rock at 9:08:40 a.m. CDT, 87-degrees above the
western horizon.

For Montgomery, Alabama it would appear
highest (66-degrees) above the east-northeast horizon at 9:11:13 a.m.
CDT. Slowing to mach 2.5 at 10:20 a.m. EDT, Discovery would be scheduled
to land at KSC at 10:26:26 a.m. EDT.

If you would like to calculate
specific viewing circumstances for your location, NASA recommends that you use
their Skywatch 2.0 applet that you can access at the space agency's space
sightings website.

When selecting a satellite, scroll to either
KSC 222 (ENTRY) or KSC 223 (ENTRY) for the options for Monday, April 19.
If there is one-day delay in the landing because of weather concerns, there are
also options for Tuesday, April 20 for a landing at either the Kennedy Space
Center: KSC 237 or 238 or for Edwards Air Force Base in California: EDW 238 or
239.

Can you see it?

If you
live very near or directly under the re-entry track, you might want to try and
view Discovery as it comes down through the atmosphere on its
northwest-to-southeast path. A reentering shuttle usually appears as a
very bright "star" leaving a long contrail in its wake.

In a
dark or twilight sky, the view of this reentry fireball can be a spectacular
sight. John A. Dormer, a Texas observer, describes the golden plasma trail
trailing behind the shuttle as "...stunning
against a rapidly-bluing sky; it looks like liquid gold."

If Discovery reenters on the first attempt (Orbit 222) it will
come just minutes before sunrise for British Columbia and Washington, meaning
re-entry comes in a bright twilight sky, but still sufficiently dark so that it
still should be a spectacular sight.

Discovery will then cross into daylight north of Idaho, where the
remainder of the reentry over the U.S. occurs during the daytime. Is it still
possible that the shuttle will be seen?

"Yes," said veteran satellite
observer, Dan Laszlo, a skywatcher based in Fort Collins, Colo. "I have
also seen shuttles on entry in daylight, appearing about as bright as Venus at
around magnitude minus 4, so it's possible, though not the spectacle of a night
entry."

"It would be hard to spot in daylight,
but not impossible," Ireland said.

Certainly, the clearer your sky with few or
no clouds and little or no haze will improve your chances of making a sighting.

Can you hear it?

If you can't see the shuttle, perhaps you'll
hear it. As it races toward Earth, a re-entering shuttle produces a
double sonic boom.

Sonic
booms are created by air pressure. Much like a boat pushes up a bow wave as
it travels through the water a vehicle pushes air molecules aside in such a way
they are compressed to the point where shock waves are formed. The reason for
two booms is that the shock waves form two cones, at the nose as well as at the
tail of the vehicle.

The shock waves move outward and rearward in
all directions and usually extend to the ground.

Those who live near and around the Kennedy
Space Center are accustomed to hearing the double booms of a returning shuttle,
but those located under and close to the Shuttle path, perhaps all the way back
to the Pacific coast, may also hear the booms as well.

From a location in the nation's midsection or
over the northwest United States, where the altitude of the shuttle will be in
the range of 100,000 to 200,000-feet, it will take time for the shock
wave to propagate down to the ground. Sound travels at roughly
1,100-feet per second, so depending on where you live relative to the track, it
could be anywhere from 90 to 180 seconds after the shuttle has passed on
by before you hear anything.

"You will definitely be able to hear the
sonic boom a few minutes after it passes directly overhead if that is where you
will be watching from," writes Texas amateur, Jeff Umbarger. "And
I once heard three booms in short succession. I can explain two of
them (the over and then under pressure wave) but not three (probably a bounce
off some thermal layer way up)."

"In 1999, one of the shuttles came in
just after sunset, on a ground track between Waco and Austin IIRC," added
Dormer. " I figured it'd be about 12 minutes before the boom hit Kyle,
Texas, which is south of Austin by 20 or so miles. I heard a
"Whump-wuh-whump" within about thirty seconds of when I thought it
would happen. There were uncertainties in my understanding of the atmosphere,
but I knew my prediction would be early."

Joe Rao serves as an instructor and guest
lecturer at New York's Hayden Planetarium. He writes about astronomy for The
New York Times and other publications, and he is also an on-camera
meteorologist for News 12 Westchester, New York.